1. Field of the Invention
The present invention relates to a method of air/fuel ratio control for an internal combustion engine.
2. Description of Background Information
In order to reduce the level of exhaust gas pollutants and improve the fuel consumption of an internal combustion engine, it is known to employ an oxygen concentration sensor to detect the concentration of oxygen in the engine exhaust gas, and to execute feedback control of the air/fuel ratio of the mixture supplied to the engine such as to hold the air/fuel ratio at a target value. This feedback control is performed in accordance with an output signal from the oxygen concentration sensor.
One type of oxygen concentration sensor which can be employed for such air/fuel ratio control produces an output which varies substantially in proportion to the oxygen concentration in the engine exhaust gas. Such an oxygen concentration sensor has been disclosed for example in Japanese patent laid-open No. 52-72286, which consists of an oxygen ion-conductive solid electrolytic member formed as a flat plate having a pair of electrodes respectively formed on two main faces, with one of these electrode faces forming part of a gas holding chamber. The gas holding chamber communicates with a gas which is to be measured, i.e. exhaust gas, through a lead-in aperture. With such an oxygen concentration sensor, the oxygen ion-conductive solid electrolytic member and its pair of electrodes function as an oxygen pump element. By passing a flow of current between the electrodes such that the electrode within the gas holding chamber becomes a negative Electrode. Oxygen gas within the gas holding chamber adjacent to this negative electrode becomes ionized, and flows through the solid electrolytic member towards the positive electrode, to be thereby emitted from that face of the pump element as gaseous oxygen. The current flow between the electrodes reaches a boundary current value which is substantially unaffected by variations in the applied voltage, and is substantially proportional to the oxygen concentration within the gas under measurement. Thus, by sensing the level of this boundary current, it is possible to measure the oxygen concentration within the gas which is under measurement. However, if such an oxygen concentration sensor is used to control the air/fuel ratio of the mixture supplied to an internal combustion engine, by measuring the oxygen concentration within the engine exhaust gas, then it will only be possible to control the air/fuel ratio to a value which is in the lean region, relative to the stoichiometric air/fuel ratio. It is not possible to perform air/fuel ratio control to maintain a target air/fuel ratio which is set in the rich region. An oxygen concentration sensor which will provide an output signal level varying substantially in proportion to the oxygen concentration in engine exhaust gas for both the lean region and the rich region of the air/fuel ratio has been proposed in Japanese patent laid-open No. 59-192955. This sensor consists of two oxygen ion-conductive solid electrolytic members each formed as a flat plate, and each provided with a pair of electrodes. Two opposing electrode faces, i.e. one face of each of the solid electrolytic members, form part of a gas holding chamber which communicates with a gas under measurement, via a lead-in aperture. The other electrode of one of the solid electrolytic members faces into the atmosphere. In this oxygen concentration sensor, one of the solid electrolytic members and its pair of electrodes functions as an oxygen concentration ratio sensor cell element. The other solid electrolytic member and its pair of electrodes function as an oxygen pump element. If the voltage which is generated between the electrodes of the oxygen concentration ratio sensor cell element is higher than a reference voltage value, then current is supplied between the electrodes of the oxygen pump element such that oxygen ions flow through the oxygen pump element towards the electrode of the element which is within the gas holding chamber. If the voltage developed between the electrodes of the sensor cell element is lower than the reference voltage value, then a current is supplied between the electrodes of the oxygen pump element such that oxygen ions flow through that element towards the oxygen pump element electrode which is on the opposite side to the gas holding chamber. In this way, a value of current flow between the electrodes of the oxygen pump element is obtained which varies substantially in proportion to the oxygen concentration of the gas under measurement, both in the rich and lean regions of the air/fuel ratio.
When such an oxygen concentration sensor which produces an output varying substantially in proportion to oxygen concentration is used for air/fuel ratio control, then a basic value for air/fuel ratio control is established in accordance with engine operating parameters relating to engine load (e.g. the pressure within the intake pipe, etc.) in the same way as for a prior art type of oxygen concentration sensor whose output is not proportional to oxygen concentration. Compensation of the basic value with respect to a target air/fuel ratio is performed in accordance with the output from the oxygen concentration sensor, to thereby derive an output value, and the air/fuel ratio of the mixture supplied to the engine is controlled by this output value. However with an oxygen concentration sensor producing an output proportional to oxygen concentration, the degree of oxygen concentration in the engine exhaust gas can be obtained from the output of the sensor. It is therefore desirable to employ an air/fuel ratio control method with such a sensor which will provide more accurate control of the air/fuel ratio than has been possible with prior art types of oxygen concentration sensor which do not produce an output proportional to the oxygen concentration. In particular, it has been hitherto extremely difficult to attain a high degree of accuracy of air/fuel ratio control during transitional engine operation such as acceleration or deceleration, due to the large fluctuations which occur in the air/fuel ratio as a result of delays in control response.